In random access information storage disk drive systems, it is conventional to provide two basic operating modes of servo control for positioning the read write head over the data tracks of the disk: track seek (or access) mode and track following mode. In the track seek mode, the head is moved rapidly over the tracks from the current track at the beginning of the seek to a target track determined from information provided by the host computer. Once at or near the target track, the system is switched to the track following mode in which the head is controlled to maintain the optical beam centered on the data track of then current interest.
During track seek, the objective is to move the head to the target track in as short a time as possible, typically through a predetermined acceleration/deceleration profile, such that the head arrives or lands at the target track with zero velocity. The seek can be as short as a single track, i.e. movement to the adjacent track, or as long as the maximum number of data tracks on the disk which typically can be as much as 10,000 or more tracks away in existing high capacity optical and magneto-optical disk drives.
In high capacity magnetic disk drive systems, it is conventional to use dedicated servo signals as inputs to the servo system for calculation of head velocity or position which is used to control the head positioning apparatus. These servo signals may be located in separate dedicated servo tracks or dedicated sectors of data tracks or may even be embedded in the data tracks themselves. An advantage of this approach is that good, clean servo signals are available to give unambiguous input data to the servo system. However, with the advent of higher capacity optical and magneto-optical drive systems, disk real estate is generally not available for prerecorded servo signals. Accordingly, in such optical and magnetooptical drive systems, servo input is typically derived from a tracking error signal produced from diffraction of the optical read/write beam caused by the transitions between the track grooves and adjacent land areas between the grooves. This tracking error signal appears as a sinusoidal wave signal as the head traverses across the tracks with the zero-crossing points on the wave corresponding to the transitions between data track grooves and adjacent land areas.
Calculation of head velocity from the tracking error signal in an optical or magneto-optical disk drive system is not as accurate as can be accomplished with dedicated servo signals due to inherently low signal-to-noise ratio and to signal corruption caused by perturbations on the disk surface. As a consequence, continuous velocity control throughout seek to the target track is not practical and a sampled servo system is usually preferred. Unfortunately, however, at low head velocity, such as is encountered at the end of seek when the head is arriving at the target track, the sampled feedback is too coarse to indicate with accuracy when the head has reached the target track where it is desired to change from velocity mode to position mode to initiate track following control. Consequently, it is desirable to have a system in which position mode control is initiated well in advance of arrival at the target track so as to bring the head to rest accurately over the target track with a high degree of reliability.